Novel insights into photoisomerization of olefins

Aromatic olefins like anthrylethylenes (la--ld) undergo one-way isomerization, which proceeds solely from the cis to trans isomer in very high quantum yields far exceeding unity, through adiabatic conversion of initially produced cis triplet ( exp 3 c*) to trans triplet ( exp 3 t*) passing through a perpendicularly twisted conformation. This is in remarkable contrast with the well accepted behavior of stilbenes and other olefins. The resulting exp 3 t*s have extraordinarily long lifetimes in the order of 10 exp 2 mu s, and therefore can, in competition with their relatively slow decay, undergo energy transfer to the cis isomer to regenerate exp 3 c*, thus accomplishing a quantum chain process. The characteristic feature of the potential energy surface was clarified with 2-(ethyl-2-d)anthracene (le--d), in which there is an energy barrier of 11 kcal/mol between exp 3 c* and exp 3 t*. The mode of isomerization of aromatic olefins, either one-way or two-way, is revealed to be governed by the triplet energy of the aromatic group on one ethylenic carbon and by the substituent on the other ethylenic C; decrease of the triplet energy of the aromatic group favors the one-way mode, and most olefins in the series of ArCH = CHPh prefer the two-way mode compared to the corresponding olefins in the series of ArCH = CH exp t Bu. 37 ref.--AA